Dynamic amplifier



2 Sheets-Sheet 2 R. C. HAWES DYNAMIC AMPLIFIER Jan. 29, 1957 Filed Dec. 5. 1951 .www

nited States Patent O l DYNAMIC AMPLIFIER Roland C. Hawes, Los Angeles, Calif., assignor to Beckman Instruments, Inc., South Pasadena, Calif., a cerporation of California Application December 5, 1951, Serial No. 259,973

7 Claims. (Cl. 324-102) This invention relates to circuits for amplifying and/ or measuring electronic signals of small magnitude and the general object of the invention is to provide a simple, reliable and inexpensive basic circuit that may be adapted to various specific purposes involving the power amplification of signals of small magnitude.

One object of the basic invention is to provide a power amplifier that functions without an input resistor. In effect, the amplifier circuit may be used as a substitute for a high-value resistor. Its utility for this purpose may be appreciated when it is considered that an exceedingly high-value resistor for use with an input current of very small magnitude is relatively expensive, as well as difiicult to fabricate with accuracy, whereas a resistor of relatively low ohmic value for use with amplified current is both inexpensive and accurate.

A further object of the invention is to provide such an amplifier that may, if desired, be embodied in a relatively small portable device energized by relatively small battery means.

An important feature of the invention is that the power of the input signal may be amplified either primarily with respect to current or primarily with respect to voltage, as may be desired.

In its application to current signals, a further important object of the invention is to provide a dynamic amplifier to serve as a rate meter for the accurate measurement of small currents from high impedance sources.

Another object with reference to current signals is to provide an exceedingly simple and rugged power amplifier that has an exceedingly high current amplification factor and in which amplification is not affected by changes in the voltage source.

A still further important object of the invention is to provide a differentiating amplifier to detect and/ or measure rates of change of an input signal. The same basic circuit that may be employed to amplify the current of an input signal characteristically responds to an input voltage signal by reflecting the rate of change of the voltage. In this aspect, the invention has utility for such purposes as measuring the rate of change of a chemical reaction and measuring the rates of change of such variables as velocity, temperature, pressure, etc.

In general, these objects are attained by the steps of placing the input control element of an amplifier in responsive communication with a signal, providing an output circuit for the amplifier with a capacitive medium of charge transfer therein, and providing coupling for feedback from the output circuit to the input.

`The capacitive medium of charge transfer permits the storage of energy in the output circuit with a voltage drop across the amplifier. A signal communicated to the input control element causes output current ow with consequent reduction of the voltage drop across the amplifier and eventually the circuit must be reset to restore the voltage drop for further operation.

In the functioning of such a circuit with feedback coupling to the input, a continuous voltage balance is 2,779,92l Patented Jan. 29, 1957 maintained in the sense that a voltage change across the capacitive medium in the output circuit constantly equals the voltage change at the input control element. Usually the feedback coupling is provided by an input capacitance. In this case, any input current applied to the input control element is amplified by a factor substantially equal to the ratio of the output capacitance to the input capacitance. On the other hand, voltage amplification may be obtained by using capacitors in series in the output circuit with the feedback taken from the output circuit at a point between the output capacitors. Thus, the power amplification of the signal may be in current amplification, or in voltage amplification, or in both. The above, and other objects and advantages of the invention, will be readily understood from the following detailed description, considered with the accompanying drawings.

In the drawings, which are to be regarded as merely illustrative:

Figure 1 is a diagram of the basic circuit;

Fig. 2 is a diagram showing how changes in the grid current vary with changing grid potential;

Figs. 3 and 4 are modifications of the basic circuit incorporating a neon tube for automatic resetting of the circuit;

Fig. 5 adds a resistance to the basic circuit to shift the range of operation on the characteristic curve of the vacuum tube;

Fig. 6 adds a cathode follower to Fig. 5;

Fig. 7 shows the basic circuit modified for response to voltage signals as distinct from current signals; and

Figs. 8 and 9, corresponding to Figs. 5 and 6, respectively, are typical circuits adapted for response to voltage signals; and

Fig. 10 is a modification of the basic circuit for power amplification by amplification of voltage in the output of the circuit.

In the basic circuit shown, by way of example, in Fig. l, the anode 10 of a vacuum tube 11 is coupled with the grid 12 of the tube by a relatively small capacitor Ci, herein termed the input capacitor. A second and much larger capacitor Ce, herein termed the output capacitor, couples the anode 10 with the cathode 15 of the tube. In this arrangement, the anode 10, the cathode 15 and the output capacitor Co form what may be termed an output circuit and the output capacitor C0 in the output path serves as a capacitive medium of charge transfer by virtue of which energy may be stored in the output circuit for current fiow through the vacuum tube It is to be understood that the vacuum tube 1i is representative of amplifiers or amplifying devices in general. Within the meaning of the term, an amplifier may com* prise a single vacuum tube or a plurality of vacuum tubes or may depend for amplification on some instrumentality other than a vacuum tube. All practices of the invention, however, will involve the use of some kind of input control element, such as the vacuum tube grid, for the control of an output circuit, and all Iforms of lthe invention will be further characterized by capacitance in the output circuit as well as coupling for feedback from the output circuit to the input. The input capacitor Ci, for example, provides such coupling.

One of the essential functions of the output capacitor C0 is to serve as a medium of charge transfer by virtue of which a voltage may be applied temporarily across the output circuit to establish therein a voltage drop across the 4amplifier with potential energy in reserve for a period of operation of the output circuit independent of the voltage source. A suitable measuring device M is placed in series with capacitor C0 to measure the current flow in the output circuit. Such a current measuring device may be,

for example, a gaivanometer or a suitable load resistance used in conjunction with a responsive vacuum tube voltmeter.

Any suitable provision may be made -for storing energy in the output circui-t when required for operation. in the particular arrangement shown, a suitable voltage source i9 is connected on its negative side with the cathode 15 and is connected .on itspositive side with the anode 1i) by a suitable switch means 20, thus lforming what may be termed a resetting circuit. The voltage source i9 may be a rectier energized by a source of alternating current, or may be a simple battery. The switching means 20 may operate automatically for periodically resetting the output circuit, but is 'here shown as a simple manually operable switch.

The switch 20 may be. `ot the normally vopen type to `be manually closed momentarily/:whenever theI potential difference across the anodeI and cathode of the vacuum tube l drops too low. On the other hand, in some practices of the invention a normally closed 'switch may be used to keep the output -capacitorCb fully charged between periods of, use, the switch being opened manually and held open whenever it is desired to place the output circuit in operation for a brief period of time.

When the resetting circuit is closed by switch 2? in the absence `of any signal to. affect 'grid 12, the grid is drawni positive by the input capacitor'Ci, and, when the grid reaches `a potential at which the sum of the grid currents is zero, plate current flows at a rate governed by the normal parameters-of lthe tube.

lf the switch 2d. is now opened to cut voltage source 19, tube 11 continues t0 draw current, but now4 the current discharges the output capacitor Co. The plate voltage falls and through the input capacitor Ci forces grid 12 negative. ,Since no-signalV is i-nuencing grid 12,. the grid shifts negatively tov cut oit vol-tage and current ceases. The output circuit is now set or readied for its intended function since the output capacitor Ca is l'eft with a substantial charge available to energize thev circuit.

'lf grid 12 is now connected toA a: high impedance signal current source with polarity to result in electron loss by the grid, the grid responds to the signal' by becoming more positive and causing output current low through the tube supplied by the charged output capacitor Ce. This plate iow acting through the input capacitor Crrnaintains grid- 12 at, an equilibrium. potential with the plate current from the output capacitor proportional tothe signal current.

ln analyzing the. action of thecircuit in the described cycle, grid current; must be taken. into consideration, as may be understood by referring' -tothe -diagrarnl of'Fig. 2. Since thev grid current is: complex, it: is: noted on the diagram inV terms of collectionor loss` of. electron-s. lt will` be noted that grid4 current isneyerreduced. to. Zero and since it is in the direction to cause plate current 4to tlow, it sets a lower limit on the4 detection: function of the circuit. This minimum is very low, however.

In region I of Figi where the grid potential is. at or near cut-oid?, the grid currents are mainly leakage currents since the grid is the most negative. element of the tube and little or no plate current is ilowing. This condition prevails when the output circuit is set. or readied. to respond to a signal, an ample'charge exis-ting: across the output condenser C0.

In response to a signal. current the grid potential changes positively to enter region Il where. many other factors contribute to the grid current, the actual shape of the grid current curve depending. on such factors. as geometry, type or emitter, gas pressure in the. tube, etc. As the grid potential .approaches4 the cathode potential, electron collection from the cathode, i. e., tube space current, becomes a majorfactor and' i'nallly. at the free grid potential indicated in Fig. 2' the collection and loss of electrons 'at the grid are. equal'. anda stable condition is reached. In region lll gridcurrents are predominantly electron collection.

In the operation' of the circuit in Fig. l to measure a current signal, the signal current ows into the input capacitor Ci to produce a change in voltage across the capacitor and thus place a signal on the grid 12. The rate of change of voltage between grid i2 and anode lt produces a substantially equal rate of change of voltage across Output capacitor CD. Since the rate ot change of voltage at the input capacitor is reflected by the amplitier to the output capacitor C0 and since the amount ot current available from a capacitor' at a given Voltage `drop across the capacitor varies with the capacitance, current amplication oy the circuit is substantially equal to the ratio of the capacitance of C0 to the capacitance of Ct. Thus, if Ii is the input current, the. magnitude of the output current I0 in the operating plate circuit will be C' .i I

The ratio may be quite high. 'For example, the value of Ci may be approximateiy ten rn-icro-microiarads andthe value of Co one-tenth of a microfarad to give a ratio of. about ten thousand. Capacitor C@ should be designed for' low leakage since such leakage reducesaccuracy.

lt. can readily be appreciated from the foregoing that the flow of current in. the output cir-cuit is a dynamic measurement and therefore the circuit operates. as rate meter reilecting changes in magnitude of the input signal current.V

Over a wide range of values'the plate characteristics of the tube need not be considered in manyv practices of the invention, but the fact that the plate voltage varies in the course of a measurement should be kept in. mind. If a triode tube is used the responsiveness of the grid to falling plate voltage will' necessitate a compensating correction in reading the value of M. The. need for such correction may be reduced by usi-ng a screen grid tube with the screen held ata constant. potential or by using a well-designed pentode, but even'the substitution of a pentode does not eliminate the need for such correction when a high degree of accuracy is, sought.

There will be some lower limit of plate voltage below which the circuit fails to function inthe manner` described, but it is contemplated that the output circuit. will be periodically reset toV keep theplate voltage above the lowerv limit. The plate voltage-maybe monitor-edwith meansv responsive to the. plate. voltage. to indicate 'when resetting of the circuit is necessary.

ItV is also. practical to. addrneans. such-1 as ancon tube to reset the outputcircuit inl response. to. the dropping. of the. plate voltage to arpredetermined. value above. the aforesaid lower limit. Such an. arrangement is shown in Fig. 3, as will now be explained.

Fig. 3 includes a tetrode 20 having. an anode 2,1, screen grid 22, control grid 23 and cathode 24. For stabilization the screen grid. 22 is connected between two` resist.- ances 27 and 28 placed in series acrossthe voltage supply 30. As in the previously described circuit,the input capacitor Ci is. connected between the anode'. 2i andthe control grid4 23. The output circuit includes ar resistance 32 and an output capacitor C0 in series with.- the anode 21, this circuit. being completed by thetworesistances 27 and 28. A suitable current measuringy device: M. may be placed in this circuit, Fig. 3 showingl the device M between the cathode 24 andl the resistance 28a To serve as an automatic switch, a,- glow discharge tube 35, in. this instance a neon tube,7 is. connected' across capacitor C0. This tube 35 becomes conducting whenever the potential ditlerence across its two. electrodes rises4 to the particular tiring potential of the tube.. As. soon as the tube fires, however,i thepotential diiierence: across the. tubey drops immediately because. of the presence of impedance; in series in the. circuit, which1impedanceinarmeni cludes the resistance 32 as well as the resistance offered by the vacuum tube 20. It is contemplated that these impedance values in the circuit will be such that when the neon tube 35 res, the potential difference across the tube will immediately drop below the extinction potential of the tube and the tube will stop conducting.

The current ow in the output circuit caused by the potential diierence between the anode 21 and the cathode 24 progressively lowers the potential of the anode and correspondingly increases the potential drop both across the capacitor C and across the neon tube 35. Eventually the neon tube lires and thereby resets the output circuit by discharging the capacitor C@ and correspondingly increasing the potential drop across the cathode and anode. This action occurs as often as necessary to keep the output circuit suiiiciently energized for its function of measuring the signal current that is communicated to the control grid 23.

The circuit shown in Fig. 4 is closely similar to the circuit shown in Fig. 3, as indicated by the use of corresponding numerals to indicate corresponding elements of the two circuits. This second circuit differs from the circuit in Fig. 3 in the omission of the resistance 32 and the insertion of a resistance 37 in series between capacitor C0 and the anode 21 of the vacuum tube.

Each of the circuits shown in Figs. 3 and 4 may be provided with a switch 38 in parallel with the neon tube and capacitor C0, if desired. Such a switch may be used to energize the output circuit independently of the neon tube 35 and m-ay be used to warm up the vacuum tube 2i) in preparation for a period of operation.

Fig. 5 shows a circuit arrangement similar to Fig. l as indicated by the use of corresponding numerals to designate corresponding components.

The purpose of Fig. 5 is to show how a suitable resistance 40 may shunt switch 20 to cause a small amount of current to flow constantly through the ampliiier tube 11 from the voltage source 19. Such a resistance of suitable value may be added to shift the range of operation of the tube 11 away from the toe of the characteristic curve of the tube to a more favorable slope on the curve.

Fig. 6 illustrates how the circuit of Fig. 5 may be modified by applying the principle of the cathode follower to the output circuit. Basically, Fig. 6 is similar to Fig. 5 as indicated by use of corresponding numerals to designate corresponding components. In Fig. 6, however, tube 11 serves as an input tube of an amplifier, the amplifier including a second tube which may be termed an output tube. connected to the grid 46 of the output tube 45, as shown, and the anode 47 of the output tube is connected directly to the positive side of the plate supply voltage 19. The cathode 48 of the output tube is connected to the negative side of the voltage supply 19 by an output capacitor C0 in series with a suitable current measuring device M. A suitable cathode resistance 50 connects the cathode 48 directly with the negative side of the plate supply voltage 19 as well as with the cathode 15 of the input tube 11.

Because the resistance 50 is on the cathode side of the output tube 45, the cathode 48 follows changes imposed on the grid 46. Since the potential difference between the cathode 48 and the grid 46 is constant, the circuit arrangement in Fig. 6 functions in the same general manner as the circuit arrangement shown in Fig. 5, there rl`he anode 10 of the input tube 11 is being the same functional relationship between the input capacitor Ci and the output capacitor C0. In fact, because of the close coupling between the grid and the cathode of the cathode follower, one terminal of Cr may be connected to the cathode of the cathode follower instead of to the cathode follower grid and the plate 1i) of the input tube.

One important advantage of the form of the invention shown in Fig. 6 is that the input tube 11 is required merely to control the grid 46 of the output tube 45 and is not taxed by the relatively large current displaced through the output capacitor Co. Since the input tube is independent of the output circuit to this extent, it may be operated under its most favorable conditions and at the same time the output capacitor C0 may be increased in size to create greater output current than would otherwise be possible within the limitations of the input tube 11.

Another feature of the circuit in Fig. 6 of importance in some practices of the invention is the low output impedance.

The remaining Figs. 7 to l0 show the basic circuit in various adaptations for responding to changes in a voltage signal as distinguished from a current signal. In this capacity, as heretofore stated, the circuit serves as a differentiator since it measures the rate of change of magnitude of the input signal rather than magnitude alone.

The circuit of Fig. 7 for voltage response corresponds to the circuit of Fig. 1 for current response, and, in the same way, Figs. 8 and 9 correspond, respectively, to Figs. 5 and 6. Corresponding numerals are employed to designate corresponding components of the circuits. It will be noted that in each instance the change required for voltage response consists in providing two input terminals instead of one, with the grid of the amplifier connected to the positively changing terminal and the input capacitor Ci connected to the negatively changing tei'- minal.

In response to voltage signals, the rate of change of input voltage is impressed directly between the grid and plate and is reflected by the amplifier tube to the capacitor C0. The relation between input voltage v and Io is expressed by Ion-Ca'itZ where dv Ff is volts per second change of the signal.

Fig. 10 shows another means whereby power amplification in the output may at least in part take the form of voltage amplification. If desired the amplification may be substantially entirely voltage amplification.

In Fig. l0, the grid 55 of an amplifier tube 56 is adapted for positively changing response to a voltage signal in the usual manner and the cathode 57 of the tube is connected to the negative side of a plate supply voltage 58. The anode 59 of the tube may be connected periodically with the positive side of the voltage supply 58 by means of a normally open switch 60.

The output circuit in Fig. l0 includes two capacitors Co and Co and a suitable measuring device, M, is shown shunting the two capacitors, although it may be connected in series therewith much as the meters M are connected in series with the capacitors C0 in Figs. 7 to 9. The output circuit is coupled with the input signal through an input capacitance Ci with the point of connection between the two capacitors Cs and C'H as shown.

Co in Fig. l0 corresponds to C0 in the previously described circuits. For high voltage gain, C"0 is preferably much smaller than C'o. The displacement current for both capacitors C0 and Co is the same but the rates at which the voltages change across the two capacitors individually are inversely proportional to their capacitances. The combined voltage change across the two is given by the equation CII C! AE 1,: AEGI n-D where AEp=change in plate voltage and AE'0=change in voltage across COO.

77 the. discussion-of 'the-.various cnrcuiis to point, stray input capacitance hasbeen-ignored-.butgof course,

it is alwaysipresent tosomedegree. In some practices oi'V the invention formeasuring changes: in current signals', stray' capacitance: may bey relied upon entirely for the function of- Ci. Thus, in'V Figs. I- and'3- to`7,V Cr may be eliminated.

Also, in Fig. l the capacitor Ci may be eliminated because capacitive: coupling isprovided' by: the capacitors C'o; and C"`0, the negatively changing input terminali being in this case connected directly to the circuit point between Cfo and Cfo.

The above, description. in. detail of preferred practices of the.inventionwill-suggest to' thosevskilled inthe art various. changes, substitutions,v and'. other departures from the specific circuits.

. L claim` as'myinvention.:

l. A device for the measurement of an electrical signal comprising the'combination of: a-signal input circuit including a tetrod'e vacuum-tube havinga signa-l input capacitance4 and: a-resi'stancey connected in series between the control. grid and. the anode ofv said' tetrode;v an output circuit. comprisinga seriesy circuit including the cathode of said tetrode,. a. source of plate Voltage supply, a capacitance, and. said: resistance in: said input circuit; a tube stabilizing circuit comprising a series circuit including a pair of resistances, saidi last-named seriescircuit being connected across said plate voltage supply, and the screen grid of said tetrode being connected between saidI resistances in .series;.a current measuring device connectedin series in said output circuit; and' switch means connected in parallelwith said capacitance in said output circuit for setting a-nvinitial charge on said V lastnamed capacitance and readying said device for operation.

2. A device as dencd in claim 1 in which the capacitance in said output circui'tis large relative to that of the capacitance in said signal input circuit.

3. In a device for measuring a small input current signal, the combination of: a vacuum amplifier tube having a control grid and an. anode; a plate voltage supply; circuit means connecting said anode to the positive. terminal of said supply; a signal input capacitor connected in series between said control grid and a point onv said circuit means; meansA for applying said' input current to said control lgrid and said1 input capacitor; an output capacitorelectrically coupled' directly" to said; point on saidl circuit meansA wherebysaid output. capacitor con tinuously undergoes' voltage changes equalling voltage changes across said input capacitor; current measuring means in series with said: outputcapacitor; and switch means'between said point on said circuit means and said positive. terminal, whereby said' switch meansv on being closed sets the charge on said input and output capacitorsV at anl initial condition and on being openedl permits said input current to alter the charge on said capacitors, the progressive change oi?y charge on saidl output capacitor being indicated by said current measuring means.

4. In a device for measuring a small input current signal, the combination of: an; input signal circuit includingva vacuum tube having acontrol grid and ananode;

S a'pl'ate voltage supply; connecting means including switch mea-nsfor connecting the positive terminal of' said supply tot saidanodea signa-l input capacitorconnectedi in series betweerrsaidv grid anda point on-sai'dconnecting means; an output circuit including an output capacitor and acurrent measuring device in seriesgvand circuit means connecting 'said anode-to. said output circuit to control the current in saidoutput circuit, said-l circuit means comprising. kmeanslelectrically coupling one ofvk the terminals of said output capacitor directly to a1pointbet-ween said switch means and? saidy anode.

5. A. device for measurement of an electrical signalcomprising the combination of: a signal input circuit including a vacuum amplier tubehaving a signal input capacitor connected between the. control grid and the anode of' saidy vacuum tube; a plate voltage supply; an output circuit comprising a series circuit including an output capacitor and a current measuring device, said series circuit being connected across: said supply; means electrically couplingone terminal of said output capacitor directly tol said anode;v and switch means between said anode and the positive: terminal of said supply for isolating said' anode. from said supply to permit. the voltage across' .said output. capacitor continuously to follow changes in potentialof saidy anode..

6. In a device for measuring a small input current signal, the combination of an input signal circuit including avacuum tubeohaving. a control grid and an anode; a'plate voltage supply;- connecting means including switch means; for connecting the; positive' terminal of said supply to said anode; a ysignal input capacitor connected in series between said gridand said anode; an output circuit includingan output: capacitor and a current measuring device in series; and circuit means connecting said anode toV said. output circuit' tol control the current in said output circuit, said circuit. means comprising. means electrically coup'ling: one of. the terminals of said output capacitor directly/.to apoint between said `switch means and saidanode.

7L A device for'. measurement. of an electrical signal comprising the; combination of: av signal input circuit including a vacuum amplier tube having a signal input capacitance connected between the control grid and the anode. ofA said vacuum tube; a plate voltage supply; an output circuitv comprising a series circuit including the cathode of said vacuum-tube, a current measuring device, a capacitance and. said anode, said series circuit being connected across said supply; and switch means connected. betweensaidanode and said supply for setting an .initial charge` condition onz said input capacitance and onI said capacitance in said output circuit.

rReferences Cited inthe le of this patent UNITED STATES PATENTS 2,019,769 Poole Nov. 5, 1935 2,086,965 Shepard July 13, 1937 2,434,297 Test et al. Jan. 13, 1948 2,562,913 Heeren Aug. 7, 1951 2,673,329 Frommer Mar. 23, 1954 

